Polymorphism phenomenon of the naphthyl-containing N-salicylidenaniline derivative: syntheses, crystal structures, optical properties, and theoretical calculations†

In the present study, we report on the synthesis and an in-depth investigation of two polymorphic modifications of the naphthyl-containing N-salicylidenaniline derivative, namely N-naphthyl-3-methoxysalicylideneimine (HL). In particular, a direct condensation reaction of 3-methoxysalicylaldehyde with 1-naphthylamine in the ethanolic medium allowed to produce crystals of polymorph I of HL of the monoclinic space group P2₁/c, while a mixture of HL and Zn(OAc)₂ in the ethanolic medium yielded crystals of polymorph II of HL of the orthorhombic space group Pbca upon slow evaporation of the solvent. Furthermore, a solvent-free grinding reaction of 3-methoxysalicylaldehyde with 1-naphthylamine also produced polymorph I of HL. In the literature, a polymorph of HL, crystallized in the monoclinic space group P2₁/c, was reported; however, this polymorph contains two molecules of HL in the asymmetric unit and was obtained by recrystallization from MeOH. Additionally, in the literature, the other polymorph of the triclinic space group P with four molecules in the asymmetric unit has also been reported, which was produced through solvent-free grinding or by employing acetonitrile in the recrystallization process. The solids synthesized in this study were characterized using IR and ¹H NMR spectroscopy. Their crystal structures and phase purity were determined via single-crystal and powder X-ray diffraction, respectively. The crystal packing of both polymorphs was further analyzed in detail through Hirshfeld surface analysis. To evaluate the optical properties of the compound, diffuse reflectance spectroscopy (solid state) and UV–vis spectroscopy combined with spectrofluorimetry (solution state) were employed. Additionally, density functional theory (DFT) calculations were conducted to provide deeper insights into the structural features and optical behavior of HL.